Typically, vehicles are fitted with an air conditioner. The air conditioner is designed to force outdoor air into the vehicle. In doing so the indoor air of the vehicle is circulated by a fan and subjected to heat exchange, thereby cooling the air.
The outdoor air contains many various fine solid pollutants, such as dusts, particulates, and pollen grains. Furthermore, with the increase in the number of vehicles the environment, in particular the air, has become polluted with various pollutants, such as particulates and harmful gases. Also, various gaseous pollutants existing in the outdoor air include a malodor.
By preventing the pollutants from entering the inside of the vehicle, the concentration of pollutants can be reduced. In doing so, an air conditioner is generally provided with a filter, thereby maintaining a comfortable indoor environment. In the conventional air conditioner, such a filter is typically mounted between a blower unit and an air discharge passage. The typical filter is configured in a cartridge fashion. In this configuration the filter can be easily exchanged following removal of a lid of a glove box on the front passenger's seat side of the vehicle.
However, a drawback is that the filter is inevitably concealed beyond the glove box due to the typical vehicle layout. Therefore, the driver often has little idea of the position where the filter is mounted, and even if the driver knows, he/she has difficulty in checking whether or not the filter is contaminated.
In order to overcome this disadvantage, several approaches have been proposed in the art. For instance, some approaches have been designed to provide the filter with an optical sensor. In one such approach, when the filter becomes contaminated with pollutants, an optical sensor causes a light receiving part to cut off a signal and thus, a warning lamp is flashed. In another approach, when the filter becomes accumulated with pollutants, an air pressure difference is generated across the filter which causes a light source to be displayed indicating the filter needs checking.
Some different approaches have been designed to provide the filter with an airflow sensor. According to one design, when the filter becomes contaminated with pollutants, an air flow sensor detects this condition and a warning lamp is flashed. Another design is constructed so that when the filter is contaminated with pollutants, an intake air pressure difference is generated across the filter, and thus a conductive thin film is fluctuated to enable a warning lamp to be flashed.
However, these conventional approaches have drawbacks in that each has a relatively complicated construction. Therefore, production costs are increased due to a circuit structure having various types of sensors, warning lamps and so forth. Furthermore, assembly time is increased.